Acute respiratory distress syndrome (ARDS) is a challenging problem in critically ill patients, which can arise from a variety of both direct and indirect injuries to the lung. The common result of these injuries is pulmonary congestion, reduced aeration, and decreased pulmonary compliance, resulting in a severe hypoxemia that can be refractory to conventional means of support. While ARDS historically carried a mortality greater than 50%,1 more recent studies have demonstrated improving outcomes,2 although the precise mortality is uncertain as observational studies report higher mortality than randomized trials.3 Although no single intervention is likely responsible for the decrease in mortality over time, there are three management techniques within the literature that have clearly demonstrated benefit.
The first, which has the broadest and best data to support it, is the use of low tidal volume (or “lung protective”) ventilation, under the rationale that lower tidal volumes are less likely to elicit barotrauma to the alveoli. The multicenter ARMA trial randomly assigned 861 mechanically ventilated patients with ARDS to receive conventional mechanical ventilation (12 ml/kg) or low tidal volume ventilation (6 ml/kg).4 This trial demonstrated an improvement in mortality (31% vs. 40%) and more ventilator free days with the low volume ventilation strategy. Subsequent to this study, several additional trials have been done, with a recent meta-analysis5 demonstrating improved mortality with low volume ventilation compared with conventional ventilation.
Another strategy to improve outcomes in ARDS has been prone positioning. Physiologic studies have demonstrated that prone positioning reduces areas of over-inflation with mechanical ventilation, while promoting alveolar recruitment.6 Several observational studies initially showed inconsistent results of prone positioning for patients with ARDS, but in 2013, the PROSEVA study group reported a multicenter prospective trial with randomized 466 ARDS patients to prone-positioning sessions of 16 hours versus being left in the supine position while undergoing low tidal volume ventilation.7 This study demonstrated a reduction in 28 day mortality from 32% to 16% in the prone group, with no significant increase in the incidence of complications.
Finally, the use of extracorporeal membrane oxygenation (ECMO) has emerged over the past several years as a powerful tool for assisting in the management of patients with ARDS. Whereas historical results for ECMO in adult respiratory disease were dismal,8 significant improvements in ECMO-associated technologies (specifically membrane oxygenators and centrifugal pumps) have led to a recent resurgence of the technology. Extracorporeal membrane oxygenation gained special attention during the 2009 H1N1 outbreak, with ECMO use associated with dramatically improved outcomes (23% mortality vs. 52% mortality).9 Similarly, the CESAR trial in the UK demonstrated a survival benefit for patients transferred to ECMO capable centers.10 Extracorporeal membrane oxygenation use allows for earlier and more aggressive lung protection ventilation strategies, facilitating the recovery from the initial injury.
With the proven benefits of prone positioning and utilization of ECMO in ARDS, the natural extension is to examine the benefit of using both techniques. There have been several small studies demonstrating improvement of oxygenation and lung compliance with the combination of the two techniques,11,12 but the fear of catastrophic complications associated with cannula dislodgement has prevented widespread adaption of treating ARDS with ECMO and prone positioning.
It is in this context that Dr. Sharma et al.13 present their current paper. It is a small case series examining the use of automated rotational percussion beds and bronchoscopy on patients with ARDS, which demonstrated an improvement in oxygenation and compliance in patients on ECMO after initiation of a rotational percussion bed (ARPB).
The authors suggest that the ARPB and repeated bronchoscopies improved pulmonary toilet and secretion management were responsible for the patient improvements. Although this is certainly a logical argument, secretion management and mucous plugging has never manifested as a serious problem in other, larger ARDS trials. Similarly, despite the logic of the rationale, serial bronchoscopy has not been demonstrated to be significantly improved more than endotracheal suctioning in either secretion management or clearance of pneumonia.
Another explanation for their improvement in physiologic parameters, in line with published data, would be that the ARBPs serve to recruit additional alveoli in a manner similar to prone positioning. This would still represent a significant improvement over proning in the ECMO population, as the risk of accidental decannulation would be substantially less. Additional studies will help elucidate both the mechanism of physiologic improvement, as well as ensure that these improvements in compliance and oxygenation translate into better patient outcomes.
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